1. Field of the Invention
The present invention relates to a carbon-fiber-based gas diffusion layer (GDL) having structured hydrophilic properties, which is used in polymer electrolyte membrane (PEM) fuel cells (FC).
2. Description of the Related Art
Fuel cells are highly efficient sources of energy in which fuel (typically gasses rich in hydrogen) and an oxidation agent (typically atmospheric oxygen) are electrochemically converted to water in catalyst layers on the two primary sides of a PEM. The electromechanical reaction generates electrical energy and causes the reactants to react, producing water. In this process, the PEM handles proton transport between the half cells, where the electrons are used via the electrodes and an external circuit to perform electrical work. The main problem of this type of fuel is cells balancing between the necessary moisture retention of the membrane and the bordering catalyst layer as well as conducting away the water produced, which otherwise fills the pores of the GDL, thereby impeding gas transport and reducing power output. The moisture level can be influenced and controlled by a series of factors. Moistening the anodic and cathodic reaction gas is often used as a means for setting the necessary moisture level. The working conditions (cell temperature, gas pressure and gas stoichiometry) are additional influencing parameters that influence the water content of the membrane and the GDL. Finally, the power range in which the fuel cell works also has a significant influence in balancing the water volume in the cell. Operating at high current densities causes a large volume of water to be produced, which must be controlled and guided so that optimal reliability and trouble-free operation is achieved.
A gas diffusion electrode with a layer for controlling the cross-diffusion of water is known from document DE 102 60 501 A1. In this example, a water cross-diffusion layer made of hydrophilic fibers, in particular hydrophilic carbon fibers, is provided between the microporous GDL and the macroporous GDL of the gas diffusion electrode.
A heat and water management device for PEM fuel cells is known from the document WO 2007/139940 A2. The device comprises a thermal and electrically conductive, hydrophilic heat and water transport element, which extends from the interior to the exterior of the cell and is arranged between the GDL and the current collector. The transport element is made of boron nitride, aluminum nitride, copper-, aluminum-, nickel-alloys, graphite, expanded graphite, graphite fabric, graphite paper, aluminum foam, stainless steel foam, nickel foam, polyvinyl alcohol foam, glass microfiber, wool fabric, cotton paper, cotton fabric, polyurethane foam, cellulose acetate, polyvinyl pyrrolidone or polyacrylamide.
What is needed in the art is a GDL configured to ensure effective water management and with which fuel cells can be controlled and operated in a simple manner even at continuous high power output.